The present invention relates to a method for manufacturing CRT (cathode-ray tube) interior parts which includes a step of forming a blackened film mainly consisting of Fe.sub.3 O.sub.4 (triferric tetraoxide) on a surface of each of CRT interior parts such as a shadow mask, an aperture grilles a frame for supporting them and an internal magnetic shield, and to a manufacturing device for implementing this method.
Two methods are known for producing a blackened film on a surface of steel parts, i.e., a steam treatment method when using a steam atmosphere and a gas treatment method using an exothermic gas. However, when using the steam treatment method, it is necessary to form a blackened film of at least 5 .mu.m! thickness in order that the blackened film can provide enough and stable protective function. Moreover, the blackened film may contain not only Fe.sub.3 O.sub.4 but also Fe.sub.2 O.sub.3 (diferric trioxide), or may contain a larger proportion of Fe.sub.2 O.sub.3 than Fe.sub.3 O.sub.4, and blackened film powder may drop off to cause clogging of the shadow mask. The gas treatment method is, therefore, largely used for important CRT interior parts, since it gives very little Fe.sub.2 O.sub.3 and forms a highly protective thin film of approximately 0.3 .mu.m! thickness.
FIG. 2 is a block diagram showing the construction of a conventional manufacturing device of CRT interior parts. In such a conventional manufacturing device, a DX gas generator for non-oxidative annealing of iron, steel, non-ferrous metals or alloys was used to manufacture steel parts such as CRT Interior parts. The manufacturing device of FIG. 2 produces an exothermic gas 13 in a conversion furnace (combustion chamber) 3 from a gaseous mixture of hydrocarbons 11 and air 12 mixed in a mixer 1. The exothermic gas 13 is then cooled to approximately 5 .degree.C.! and moisture is removed in a cooler 8 to give a DX gas 15. The DX gas 15 passes through a hot water shower at 60 .degree.C.! generated by a humidifier 9, and after being reheated to approximately 500 .degree.C.! (oxidizing temperature) by a heater 10, it is led into a blackening furnace 7 containing CRT interior parts.
However, in the conventional manufacturing device of CRT interior parts, a large energy loss was involved as the DX gas 15 formed by cooling to 5 .degree.C.! and dehumidifying is re-humidified and reheated.
Further, the humidifier 9 requires difficult maintenance such as rustproofing.
Further, due to a shift of the dew point resulting from instability of the humidifying conditions produced by the humidifier 9, there was dispersion in the thickness and properties of the blackened film.